Optical whispering-gallery-mode resonators (WGMRs) have emerged as promising platforms for label-free detection of nano-objects. The ultimate sensitivity of WGMRs is determined by the strength of the light-matter interaction quantified by quality factor/mode volume, Q/V, and the resolution is determined by Q. To date, to improve sensitivity and precision of detection either WGMRs have been doped with rare-earth ions to compensate losses and increase Q or plasmonic resonances have been exploited for their superior field confinement and lower V. Here, we demonstrate, for the first time to our knowledge, enhanced detection of single-nanoparticleinduced mode splitting in a silica WGMR via Raman gain-assisted loss compensation and WGM Raman microlaser. In particular, the use of the Raman microlaser provides a dopant-free, self-referenced, and self-heterodyned scheme with a detection limit ultimately determined by the thermorefractive noise. Notably, we detected and counted individual nanoparticles with polarizabilities down to 3.82 × 10 −6 μm 3 by monitoring a heterodyne beatnote signal. This level of sensitivity is achieved without exploiting plasmonic effects, external references, or active stabilization and frequency locking. Single nanoparticles are detected one at a time; however, their characterization by size or polarizability requires ensemble measurements and statistical averaging. This dopant-free scheme retains the inherited biocompatibility of silica and could find widespread use for sensing in biological media. The Raman laser and operation band of the sensor can be tailored for the specific sensing environment and the properties of the targeted materials by changing the pump laser wavelength. This scheme also opens the possibility of using intrinsic Raman or parametric gain for loss compensation in other systems where dissipation hinders progress and limits applications.optical sensor | active microresonator | particle sensing T here is an increasing demand for new technologies to detect small molecules, nanoparticles, and airborne species (1-4). In the past decade we have witnessed a boost in the number of label-free detection techniques with varying levels of sensitivities. Techniques relying on electrical conductance (5), light scattering and interferometry (6-8), surface and localized plasmon resonance (9, 10), nanomechanical resonators (11,12), and optical resonances (13-17) have been demonstrated.Whispering-gallery-mode (WGM) microresonators with their high quality factor, Q, and small mode volume, V, are known to enhance light-matter interactions and have extraordinary sensitivities to changes and perturbations in their structure or proximity (18,19). They have been of great interest for sensing biomarkers, DNA, and medium-size proteins at low concentrations, as well as for detecting viruses and nanoparticles at single-particle resolution (19-31). A particle or molecule entering the mode volume of a resonator or binding onto its surface induces a net change in the polarizability of the resonator-...

Summary Optical whispering-gallery mode (WGM) microresonators, confining resonant photons in a microscale volume for long periods of time, strongly enhance light-matter interactions, making them an ideal platform for photonic sensors. One of the features of WGM sensors is their capability to respond to environmental perturbations that influence the optical mode distribution. The exceptional sensitivity of WGM devices, coupled with the diversity in their structures and the ease of integration with existing infrastructures, such as conventional chip-based technologies, has catalyzed the development of WGM sensors for a broad range of analytes. WGM sensors have been developed for multiplexed detection of clinically relevant biomolecules while also being adapted for the analysis of single-protein interactions. They have been used for the detection of materials in different phases and forms, including gases, liquids, and chemicals. Furthermore, WGM sensors have been used for a wide variety of field-based sensing applications, including electric field, magnetic field, force, pressure, and temperature. WGM sensors hold great potential for applications in life and environmental sciences. They are expected to meet the ever-increasing demand in sensor networks, the Internet of Things, and real-time health monitoring. Here we review the mechanisms, structures, parameters, and recent advances of WGM microsensors and discuss the future of this exciting research field.

Infrared spectroscopy has found wide applications in the analysis of biological materials. A more recent development is the use of engineered nanostructures – plasmonic metasurfaces – as substrates for metasurface-enhanced...

Figure 4. High-Q TiO 2 WGM microgoblet resonators experience resonance narrowing (A) and broadening (B) when the wavelength of an input laser is up-and down-scanned, respectively. (C) Change in the resonance wavelength of a TiO 2 WGM microresonator when the temperature of its surrounding is increased. (D) Thermally induced fast and slow oscillations due to the interplay between slow thermal expansion, which red-shifts, and the faster thermo-optic effect, which blue-shifts the resonance wavelength. Here wavelength of the laser is not scanned. In (A), (B) and (D), a.u denotes arbitrary units. wileyonlinelibrary.com